1. Field of the Invention
The present invention is related to the field of atmospheric studies. In particular, the invention is related to use of the GPS constellation to determine atmospheric conditions.
2. Description of the Related Technology
GPS receiver arrays have previously been employed for mapping tropospheric water vapor and the total electron content (TEC) of the ionosphere. The receiver arrays track all GPS satellites above a low-elevation threshold, typically ˜10° above the horizon. Originally, only maps of the total column water vapor, or TEC, were obtained from relatively sparse arrays. Dense GPS arrays permit tomographic mapping of water vapor content in the troposphere. Tropospheric water vapor tomography exploits the time delays of the GPS signals between all satellite-receiver pairs. The delays are measured and preprocessed to remove the systemic errors from both the satellite and the receivers. Inverting the preprocessed data set produces a map of tropospheric water vapor.
Typically, GPS arrays cover from 102-105 km2 and employ 15-70 individual receivers. Accurate atmospheric measurements require precise GPS orbits, and these are typically available within 1-3 weeks of data collection. Smaller arrays can be sensitive to the low-elevation cut-off. In all cases, GPS arrays are benchmarked at known, fixed locations.
While long-baseline GPS arrays have been developed to make atmospheric measurements, short-baseline GPS arrays have been developed for attitude (roll, pitch and yaw) determination of surface vehicles, aircraft and Low Earth Orbit (LEO) satellites. Short-baseline arrays typically consist of 2-4 receivers with baselines between antennas of 0.1-10.0 m. The short-baseline arrays are designed for dynamic position and attitude determination. Hence the absolute array location follows from standard GPS algorithms with the inherent GPS user equivalent range errors, 1σ˜1-2 m. User equivalent range errors can arise from uncertainties in the GPS satellite clock and satellite ephemeris, and, at low-elevation angles, unknown tropospheric water vapor content.
Attitude derives from precise relative locations of the array antennas with respect to a reference antenna. Determining relative positions of the array antennas to an accuracy of ˜0.2 mm is achieved by tracking the phase of the L1 GPS carrier frequency (f=1575.42 MHz or λ≈19 cm) that is employed by commercially available GPS receivers. For small arrays, the atmospheric propagation delays are the same for all receivers. Therefore, for interferometric measurements, the troposphere, ionosphere and systematic satellite induced range errors cancel. While the absolute distance between satellite i and receiver j, φji, is not precisely determined, the difference, φji−φki, between the satellite i and the two receivers j and k is known to a fraction of a wavelength. For small baselines the GPS antennas all employ the same receiver clock, therefore, receiver clock errors are insignificant. For a 1 meter interferometric baseline the relative positional accuracy of 0.2 mm translates into an angular accuracy of 0.01°. The angular accuracy is baseline dependent, thus doubling the interferometric baseline doubles the angular resolution.
Relevant ship-borne atmospheric measurements typically consist of “daily” radiosondes. These instrumented balloons provide accurate temperature, pressure and water vapor measurements along their trajectory. However, they present an uncontrolled flight path and an airborne hazard for aircraft in the vicinity. Additionally, diurnal variations, which are known to be large, are completely missed by infrequent radiosonde launches. Therefore, at present, there is no reliable method to assess local atmospheric refraction in a timely manner.
Surface-level atmospheric ducting represents a significant challenge to detection of airborne targets. However, current methods of detecting atmospheric ducting are impractical and/or inaccurate. It is therefore desirable to provide a new technique and system to assess the presence of atmospheric ducting.